#define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<cstdio>
#include<cmath>
#include<algorithm>
using namespace std;
int L, N, M; int i, j;
int arr[50010];
int ans;
int judge(int digit)
{
	int i = 0;
	int now = 0;
	int tone = 0;
	while (i < N + 1) {
		i++;
		if (arr[i] - arr[now] < digit) {
			tone++;
		}
		else {
			now = i;
		}
	}
	if (tone > M)return 0;
	else return 1;
}
int main()
{
	cin >> L >> N >> M;
	for (i = 1; i <= N; i++) {
		scanf("%d", &arr[i]);
	}
	arr[N + 1] = L;
	int l = 1;
	int r = L;
	int mid;
	while (l <= r) {
		mid = (l + r) / 2;
		if (judge(mid)) {
			ans = mid;
			l = mid + 1;
		}
		else {
			r = mid - 1;
		}
	}
	cout << ans;
	return 0;
}